This invention relates to a self-tapping screw for use in fixing insulating boards and waterproof sheeting, if such is required to be fixed with the insulating boards, to a roofing panel, especially to a sheet panel forming a coverplate for a flat roof. In particular the present invention concerns a self-tapping screw having a head, a self-drilling tip and a shank connecting the head to the tip and including a self-tapping threaded section adjoining the self-drilling tip, and at least one smooth shank section located between two threaded sections.
A self-tapping screw of the above mentioned form is disclosed in the German Auslegeschrift No. 2,126,914. In the known screw, a first threaded section directly adjoins the screw head and is followed by a first smooth shank section. All transitions between the threaded sections and the smooth shank sections are made as a normal thread run-in or thread run-out. The known screw has three different threaded sections, each of which has a different outside diameter. The threaded section adjacent to the screw head has the largest outside diameter and the thread section furthest away from the screw head has the smallest diameter. With this design, there is a risk that the continually changing wind-pressure and suction forces which act on the insulating board and possibly the waterproof sheeting(s) of a flat roof covering, will loosen the seating of the screw, turn back the screw, and finally pull the screw completely out of a drilled hole, because in the drilled hole the threaded sections having a smaller outside diameter provide lower retaining forces.